1. Field of the Invention
The present invention relates to an apparatus and method for cordless telephonic communication and, more particularly, for selecting a free channel for such communication.
2. Brief Description of Related Technology
Cordless, or wireless, telephones typically include a base station and a separate handset unit which is typically adapted to be seated in a cradle formed in the base station. The general operation of the wireless telephone is controlled by a micro controller integrated therein. Such telephones are commonly capable of operating, i.e., transmitting and receiving signals, on any one of a number of separate channels. A radio frequency (RF) module is typically utilized by the micro controller for establishing communication on a single channel, the selection of which channel is discussed below. Time division multiplexing, such as duplexing, may be utilized to both transmit and receive signals on the same channel. Transmit and receive time slots are allocated so signals are alternately transmitted and received, respectively, in the different time slots. The transmit and receive time slots are of substantially equal duration and may be non-overlapping.
To select a channel for communication, the telephone automatically samples signals from each channel to determine which channel is free, i.e., has the least amount of signal traffic on it. The free channel selection is performed according to a predefined algorithm. One example may be that each of the channels is examined, or sampled, for radio frequency activity for a period of time from between 200 milliseconds (ms) to 2 seconds. The channel with the lowest radio frequency activity is the optimum channel to be used to establish a radio link between the handset unit and the base station. The signal samples, however, are valid for only a predetermined period of time. Therefore, all channels must be sampled and re-examined repeatedly.
The free channel selection function is performed typically by the micro controller according to software instructions stored therein. Accordingly, the micro controller sets the RF module to a particular channel. In one embodiment, the signal is then sampled at least four times during a 2 ms transmit/receive window. A Receive Signal Strength Indicator (RSSI) circuit then provides the strength of the signal samples. The micro controller reads the RSSI level for the signals of each channel and, using conventional comparison techniques, selects for radio communication the channel having the lowest RSSI level.
A disadvantage of the foregoing process and system is that substantial overhead is added to the micro controller, which inhibits the micro controller from performing other system tasks. Another disadvantage of the foregoing process is that it also shortens the life of the battery in the handset unit. One solution to this drawback is to use a faster, more powerful micro controller which can manage the additional overhead. But, such a micro controller increases the cost of the telephone and increases the power requirement to operate it. Since the handset of a cordless telephone draws its power from a battery, increased power demands drain the battery faster, thereby decreasing the operating time.
Therefore, what is needed is an apparatus and method in which the free channel selection of a cordless telephone is performed without unduly burdening the micro controller and without requiring increased amounts of power.
The present invention, accordingly, provides an apparatus and method in which the free channel selection of a cordless telephone is performed without unduly burdening the micro controller and without requiring increased amounts of power. To this end, a free channel selector of the present invention comprises an A/D converter which digitizes the RSSI signal from a radio frequency (RF) module for a selected and monitored channel. The A/D converter generates, for each of a plurality of channels, a first signal indicative of the strength of the signals transmitted on the respective channel, and an end of conversion (EOC) signal. The first signal and the EOC signal are then provided to a peak detector circuit which compares the strength of the first signal to the signal strength of a previously selected channel. A memory device having an array of cells is provided for storing values representative of the signal strength of the channel having the least signal activity, where the stored values stored in a cell having an address indicative of the channel on which the first signal represented by the stored value is carried. The address of the selected stored value is indicative of the free channel.
An advantage achieved with the present invention is that it enables the use of relatively inexpensive micro controllers that require relatively little power in cordless telephones, thereby extending battery life and the operating time of the telephone.
Another advantage achieved with the present invention is that it frees up the micro controller to perform other tasks while the free channel selection process is being performed.